This invention relates to optical amplifiers, and finds particular, but not necessarily exclusive application in the construction of low gain amplifiers, amplifiers providing only a few dB of gain or less, as opposed to line amplifiers, which are typically required to provide significantly more than ten dB of gain. (For the purposes of this specification, the terms xe2x80x98opticalxe2x80x99 and xe2x80x98lightxe2x80x99 should be understood as pertaining not only to the visible part of the electromagnetic spectrum, but also to the infra-red and ultra-violet parts that bound the visible part.)
The essential components of an optical amplifier comprise an optical input port optically coupled with an output port via an optical transmission path that includes an optical gain medium, and means for powering that gain medium. In the case of known semiconductor optical amplifiers, the power is electrical power, and hence such an amplifier is provided with electrodes by which electrical power is applied. In the case of known rare-earth doped glass optical fibre amplifiers, the power is optical power, and this is conveniently applied via the input port, the output port, or in some circumstances simultaneously through both ports.
A known form of optical amplifier of more recent origin is the broadband polymer amplifier. Such amplifiers are the subject of research at St. Andrews University, Fife, Scotland, and some of the research carried out there has been publicly described by Professor I Samuel at the Rank Prize Conference on Broadband Optical Amplifiers held at Grassmere, England Jun. 18th to 21st, 2001. Amongst such polymer amplifiers are those that may be termed direct emitting polymers in order to distinguish them from other polymer amplifiers in which the polymer acts as a host for an active ingredient in an manner analogous with the way the glass matrix of a rare-earth doped glass optical fibre amplifier acts as a host for the rare-earth dopant. The above-referenced authors have described an optical set-up for investigating the optical amplification provided by these polymers in which the polymer takes the form of a thin film supported upon a planar substrate of lower refractive index. A cross-section of such a set up is schematically depicted in FIG. 1 in which the polymer film is depicted at 10 and its supporting substrate at 11. A light signal to be amplified is launched as a collimated beam 12 into a prism 13 to emerge at grazing incidence from a face of the prism in contact with the film 10. A beam 14 of pump light is directed into the polymer film 10, and the amplified signal 15 emerges from the far end of the film.
It has been found that these optically amplifying polymers are capable of exhibiting a gain per unit length (measured along the transmission path of the optical signal being amplified) that is significantly larger than that commonly exhibited either by rare-earth doped glass optical fibre amplifiers and that commonly exhibited by semiconductor amplifiers. In the case of direct emitting polymers, it is postulated that the particularly high gain per unit length results from the fact that light emission occurs from electrons within the molecular structure of the polymer itself, rather than from electrons within a dopant, and that accordingly the density of optically active sites is particularly high.
The present invention is directed to the construction of a new topology of optical amplifier made practical by the availability of amplifying media with high gain per unit length.
An undesirable feature of passive integrated optics devices, and also integrated optics devices that incorporate active components but no gain-providing components, is that they inevitably attenuate to a certain extent any signal caused to propagate through them. In many circumstances this attenuation is only of the order of 1 dB. However this can present a problem, particularly if the signal is caused to propagate through two or more of such devices arranged optically in cascade and without any intervening amplifier. This invention provides a way of compensating, at least in part, for such attenuation by integrating an optical amplifier with such an integrated optics device, converting it into a hybrid device including an active component.
According to a first aspect of the present invention, there is provided an optical amplifier having an optical input port optically coupled with an optical output port via an optical waveguide having an optical core intersected by a transverse trench occupied by a polymeric optically amplifying medium.
According to a second aspect of the present invention, there is provided a method of making an optical amplifier including the step of constructing an optical waveguide having a filamentary optical core extending from an input port of the amplifier to an output port thereof, the step of forming a transverse trench in the optical waveguide to intersect its optical core, and the step of depositing an optically amplifying polymer in said trench.
The waveguide is preferably a waveguide of a kind in which a filamentary optical core of a first refractive index is fabricated on a substantially planar supporting substrate of a second refractive index that is lower than the first, the optical core then being covered with material whose refractive index is also lower than that of the optical core (and typically substantially matched with that of the supporting substrate), such a waveguide hereinafter being referred to as a planar waveguide.
Optical pump power may be launched into the optically amplifying medium in the trench axially via the optical waveguide, but generally it may be preferred to launch that power into the medium via its side (transverse launch). This is because, under axial launch conditions, the pump power is confined to the mode(s) guided by the waveguide; whereas, under transverse launch conditions, many more modes are available. Conveniently this transverse launch may be provided by a diode pump laser located on top of the trench.
The refractive index of the polymeric material is fairly close to that of conventional planar waveguides and certainly much less than that of semiconductor materials. Therefore the intrinsic radiative (diffraction) loss involved in coupling light from the part of the waveguide on one side of the trench to the part on the other side is small compared with 0.3 dB for trench thicknesses up to several xcexcm. Accordingly trench widths of not more than a few microns can be employed to provide amplifier gains of not more than a few dB, sufficient for compensating for the insertion loss of certain types of planar waveguide passive devices, by way of example wavelength multiplexer/demultiplexer devices. Typically it may be found convenient to form the waveguide of the amplifier monolithically with the waveguides of the passive device.
Where higher valued gains are required, two or more amplifying polymer filled transverse trenches may be included optically in cascade along the length of the optical waveguide. If the gain is high enough, the cascade may need to be interrupted by one or more optical isolators to reduce stray feedback to a level sufficient to preclude laser action.